The invention relates to methods for machining flat surfaces of a workpiece using a tool, in particular a milling tool.
In spite of the simple geometry of flat surfaces, there are many different machining strategies that differ, among others, in terms of the tools used, the technical requirements, the production duration and the surface quality that can be achieved. Such flat surfaces or planes are specifically found in many parts in tool and mould construction.
For instance, face milling is often used to machine flat surfaces. A cylindrical end or radius mill, an end mill with rounded tip, is moved with the front face over the plane for this. For example, work is conducted in parallel to the contour or in lines, with consistent or alternating directions. In the area of pocket processing, as described for example in US 2013/0151000 A1 or U.S. Pat. No. 8,489,224 B2, in which a plane at the bottom defines the essential geometry, more complex path shapes such as spirals and trochoids are used as well. The tool in face milling is essentially vertical on the plane, but may be slightly inclined in the infeed direction and/or laterally to it to adjust the cutting conditions. Even though face milling has proven its worth in machining freely accessible and/or “free-standing” planes in practice, face milling has some inherent disadvantages. On the one hand, machining of planes the machining of which is impaired by adjacent surfaces is possible only with great limitations or even entirely excluded. For example, when a collision of the tool holder with a surface adjacent to the plane to be machined threatens, the extension length, i.e. the length by which the tool protrudes from the holder, would have to be increased, which impairs stability at maintenance of the tool diameter. Machining of a pocket, for example, comprising three or more adjacent flat side walls and a floor surface, by face milling is impossible, since the tool with holder and possibly further components such as the spindle, etc., will usually not fit inside the pocket. On the other hand, the tool must be pivoted by 90° when machining lateral planes of a workpiece. With a large workpiece, there often will not be enough space left in the working space of the machine to position the pivoted tool with the holder and the further components. Personnel-, time- and therefore cost-intensive re-clamping of the workpiece is the consequence of this.
As an alternative, punching is used for machining flat surfaces. In this, an end mill is usually moved in several passes along its axis vertically from the top down over the plane to be machined, whereby the tool may also be moved at a specific application angle to the plane. However, also punching has considerable disadvantages in practice. If the tool is long enough and vertical accessibility is ensured, even planes that are hard to reach in face milling can be machined. However, a tool of such length is often subject to high dislocation and the risk of unstable machining in general. If, however, a sufficiently long tool cannot be used for the plane, for example because vertical accessibility is missing, a collision of the holder cannot be avoided and complete machining is impossible. Punching also requires a very large number of vertical passes in order to achieve a certain surface accuracy, in particular since the diameter of the tool must not be too large to permit machining of corners as well.
There also is the option of swarf milling, which is mostly used for machining planes and free-form surfaces, as may be taken for example from U.S. Pat. No. 5,391,024 A and US 2015/0032250 A1. A cylindrical end mill is aligned in parallel with the plane or free-form surface to be machined for this, brought into contact with it and moved orthogonally to its axis. Thus, the flank of the tool is used predominantly. Swarf milling permits larger step widths than face milling or punching due to the resulting larger reach area. However, the swarf milling has also turned out to have comparatively high disadvantages in practice for machining planes due to the collision and stability problems. Due to the alignment of the tool in parallel with the plane, the extension length must at least correspond to the height of the plane in order to completely machine it without collision. A large extension length, however, considerably reduces stability of the tool and thus the milling quality. To improve stability, the cylindrical end mill has been replaced by a conical mill with or without spherical front face. In order to bring the tool into contact with the plane, it must be angled. The conical shape of the tool, in connection with its inclination, leads to a lower displacement, since part of the cutting forces can be discharged in the direction of the tool axis. The reduced displacement permits larger extensions. But the inclined tool holder is usually not far enough from the plane due to the usually small cone angle in order to avoid collision. If the surfaces adjacent to the plane are located detrimentally, it may be impossible to put the tool into contact with the plane without causing a collision between the holder and the adjacent surfaces. Abrupt transfers between the conical blade and the shaft or the flat or spherical front face are particularly detrimental as well. The transfers can specifically lead to undesired strip-like traces at the workpiece when the blade is completely supported, in particular if the pivot axes of the milling machine used do not work precisely enough.
Multipass milling is known as another strategy for plane machining. The tool is placed vertically or at a specific angle to the flat surface and is moved back and forth. A ball mill is used most frequently for this. Such multipass machining of flat surfaces is described for example in JP 2011-251401 A, with the ball mill being moved with a lead angle for an alternating infeed direction to improve the cutting conditions (synchronous milling). It has turned out to be particularly disadvantageous in practice for multipass milling that the tool or its spherical front face must be narrow enough to machine even narrow sections. Furthermore, a comparatively small distance between the machining passes is necessary in order to permit a specified surface accuracy. Both lead to a large number of passes and make machining accordingly long. Multipass milling for machining flat surfaces therefore is often inefficient and thus not least comparatively costly.
To solve the collision problem, which occurs in many machining strategies with different tools, JP 2011-183528 A further suggests a method where the tool is to be inclined against the surface to be machined in the respective positions of the tool path. A change to the contact point at the tool in this context is not considered and can lead to decisive disadvantages depending on the type of tool. Furthermore, it is not possible to specifically control tool application on one hand while taking measures to prevent collisions independently of this on the other hand. The tool inclination also serves exclusively to prevent collisions with the flat surface to be machined itself, but not with any adjacent surfaces.
As a result, these known milling methods have considerable disadvantages regarding the technical feasibility, the options for avoiding collision, the stability and the efficiency, as well as the cost effort connected to this.